1. Technical Field of the Invention
The invention relates to an optical pickup device, and more particularly to a device suitable for use in irradiating a recording medium having plural laminated recording layers with laser light.
2. Disclosure of Related Art
In recent years, as the capacity of an optical disc has been increased, an optical disc having an increased number of recording layers has been developed. Laminating recording layers in a disc enables to considerably increase the data capacity of the disc. In the case where recording layers are laminated, generally, two recording layers are laminated on one side of a disc. Recently, however, laminating three or more recording layers on one side of a disc has been put into practice to further increase the capacity of the disc. Thus, the capacity of a disc can be increased by increasing the number of recording layers to be laminated. However, as the number of recording layers to be laminated is increased, the distance between the recording layers is decreased, and signal deterioration resulting from an interlayer crosstalk is increased.
As the number of recording layers to be laminated is increased, reflection light from a recording layer (a targeted recording layer) to be recorded/reproduced is reduced. As a result, if unwanted reflection light (stray light) is entered into a photodetector from a recording layer on or under the targeted recording layer, a detection signal may be deteriorated, which may adversely affect focus servo control and tracking servo control. In view of this, in the case where a large number of recording layers are laminated, it is necessary to properly remove stray light, and stabilize a signal from a photodetector.
Japanese Unexamined Patent Publication No. 2010-102813 (corresponding to U.S. Patent Application Publication No. US2010/0080166 A1) discloses an arrangement of an optical pickup device that enables to properly remove stray light, and suppress an offset (a DC component) of a tracking error signal, in the case where a large number of recording layers are formed in an optical disc. With this arrangement, it is possible to form an area where only signal light exists, on a light receiving surface of a photodetector. It is possible to suppress an influence of stray light on a detection signal by disposing sensors of the photodetector in the signal light area. Further, with this arrangement, it is possible to suppress an offset (a DC component) of a tracking error signal resulting from shift of an objective lens with respect to an optical axis of laser light.
In the above optical pickup device, a tracking error signal is generated by subtracting, from a signal component reflecting a push-pull component, a signal obtained by multiplying a signal component reflecting a DC component with a variable k to suppress an offset (a DC component) of a tracking error signal. In the above arrangement, if noise is included in a signal component reflecting a DC component, an influence of noise on a tracking error signal may be increased.